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Date:   Mon, 18 Jul 2022 15:13:35 -0700
From:   "Darrick J. Wong" <djwong@kernel.org>
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     "ruansy.fnst@fujitsu.com" <ruansy.fnst@fujitsu.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
        "nvdimm@lists.linux.dev" <nvdimm@lists.linux.dev>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "david@fromorbit.com" <david@fromorbit.com>,
        "hch@infradead.org" <hch@infradead.org>,
        "jane.chu@oracle.com" <jane.chu@oracle.com>
Subject: Re: [RFC PATCH v6] mm, pmem, xfs: Introduce MF_MEM_REMOVE for unbind
Message-ID: <YtXbD4e8mLHqWSwL@magnolia>
References: <20220410171623.3788004-1-ruansy.fnst@fujitsu.com>
 <20220714103421.1988696-1-ruansy.fnst@fujitsu.com>
 <62d05eb8e663c_1643dc294fa@dwillia2-xfh.jf.intel.com.notmuch>
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On Thu, Jul 14, 2022 at 11:21:44AM -0700, Dan Williams wrote:
> ruansy.fnst@fujitsu.com wrote:
> > This patch is inspired by Dan's "mm, dax, pmem: Introduce
> > dev_pagemap_failure()"[1].  With the help of dax_holder and
> > ->notify_failure() mechanism, the pmem driver is able to ask filesystem
> > (or mapped device) on it to unmap all files in use and notify processes
> > who are using those files.
> > 
> > Call trace:
> > trigger unbind
> >  -> unbind_store()
> >   -> ... (skip)
> >    -> devres_release_all()   # was pmem driver ->remove() in v1
> >     -> kill_dax()
> >      -> dax_holder_notify_failure(dax_dev, 0, U64_MAX, MF_MEM_PRE_REMOVE)
> >       -> xfs_dax_notify_failure()
> > 
> > Introduce MF_MEM_PRE_REMOVE to let filesystem know this is a remove
> > event.  So do not shutdown filesystem directly if something not
> > supported, or if failure range includes metadata area.  Make sure all
> > files and processes are handled correctly.
> > 
> > ==
> > Changes since v5:
> >   1. Renamed MF_MEM_REMOVE to MF_MEM_PRE_REMOVE
> >   2. hold s_umount before sync_filesystem()
> >   3. move sync_filesystem() after SB_BORN check
> >   4. Rebased on next-20220714
> > 
> > Changes since v4:
> >   1. sync_filesystem() at the beginning when MF_MEM_REMOVE
> >   2. Rebased on next-20220706
> > 
> > [1]: https://lore.kernel.org/linux-mm/161604050314.1463742.14151665140035795571.stgit@dwillia2-desk3.amr.corp.intel.com/
> > 
> > Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
> > ---
> >  drivers/dax/super.c         |  3 ++-
> >  fs/xfs/xfs_notify_failure.c | 15 +++++++++++++++
> >  include/linux/mm.h          |  1 +
> >  3 files changed, 18 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> > index 9b5e2a5eb0ae..cf9a64563fbe 100644
> > --- a/drivers/dax/super.c
> > +++ b/drivers/dax/super.c
> > @@ -323,7 +323,8 @@ void kill_dax(struct dax_device *dax_dev)
> >  		return;
> >  
> >  	if (dax_dev->holder_data != NULL)
> > -		dax_holder_notify_failure(dax_dev, 0, U64_MAX, 0);
> > +		dax_holder_notify_failure(dax_dev, 0, U64_MAX,
> > +				MF_MEM_PRE_REMOVE);
> >  
> >  	clear_bit(DAXDEV_ALIVE, &dax_dev->flags);
> >  	synchronize_srcu(&dax_srcu);
> > diff --git a/fs/xfs/xfs_notify_failure.c b/fs/xfs/xfs_notify_failure.c
> > index 69d9c83ea4b2..6da6747435eb 100644
> > --- a/fs/xfs/xfs_notify_failure.c
> > +++ b/fs/xfs/xfs_notify_failure.c
> > @@ -76,6 +76,9 @@ xfs_dax_failure_fn(
> >  
> >  	if (XFS_RMAP_NON_INODE_OWNER(rec->rm_owner) ||
> >  	    (rec->rm_flags & (XFS_RMAP_ATTR_FORK | XFS_RMAP_BMBT_BLOCK))) {
> > +		/* Do not shutdown so early when device is to be removed */
> > +		if (notify->mf_flags & MF_MEM_PRE_REMOVE)
> > +			return 0;
> >  		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_ONDISK);
> >  		return -EFSCORRUPTED;
> >  	}
> > @@ -174,12 +177,22 @@ xfs_dax_notify_failure(
> >  	struct xfs_mount	*mp = dax_holder(dax_dev);
> >  	u64			ddev_start;
> >  	u64			ddev_end;
> > +	int			error;
> >  
> >  	if (!(mp->m_sb.sb_flags & SB_BORN)) {
> >  		xfs_warn(mp, "filesystem is not ready for notify_failure()!");
> >  		return -EIO;
> >  	}
> >  
> > +	if (mf_flags & MF_MEM_PRE_REMOVE) {
> > +		xfs_info(mp, "device is about to be removed!");
> > +		down_write(&mp->m_super->s_umount);
> > +		error = sync_filesystem(mp->m_super);
> > +		up_write(&mp->m_super->s_umount);
> 
> Are all mappings invalidated after this point?

No; all this step does is pushes dirty filesystem [meta]data to pmem
before we lose DAXDEV_ALIVE...

> The goal of the removal notification is to invalidate all DAX mappings
> that are no pointing to pfns that do not exist anymore, so just syncing
> does not seem like enough, and the shutdown is skipped above. What am I
> missing?

...however, the shutdown above only applies to filesystem metadata.  In
effect, we avoid the fs shutdown in MF_MEM_PRE_REMOVE mode, which
enables the mf_dax_kill_procs calls to proceed against mapped file data.
I have a nagging suspicion that in non-PREREMOVE mode, we can end up
shutting down the filesytem on an xattr block and the 'return
-EFSCORRUPTED' actually prevents us from reaching all the remaining file
data mappings.

IOWs, I think that clause above really ought to have returned zero so
that we keep the filesystem up while we're tearing down mappings, and
only call xfs_force_shutdown() after we've had a chance to let
xfs_dax_notify_ddev_failure() tear down all the mappings.

I missed that subtlety in the initial ~30 rounds of review, but I figure
at this point let's just land it in 5.20 and clean up that quirk for
-rc1.

> Notice that kill_dev_dax() does unmap_mapping_range() after invalidating
> the dax device and that ensures that all existing mappings are gone and
> cannot be re-established. As far as I can see a process with an existing
> dax mapping will still be able to use it after this runs, no?

I'm not sure where in akpm's tree I find kill_dev_dax()?  I'm cribbing
off of:

https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/tree/fs/xfs/xfs_notify_failure.c?h=mm-stable

--D